kcms/tabbox: Highlight non-default shortcuts
This sets the property on the shortcut widgets when they should get highlighted: the option is selected in System Settings and the shortcut is not the default.
- On top of !3417 (merged)
- Requires frameworks/kxmlgui!138 (merged) to actually paint the highlihgt ring.
NOTE: This doesn't affect the global "non-default dot" for the KCM on the SySe sidebar. That uses a different codepath without even loading the KCM (just its KCModuleData). The code of this KCM is now a bit old and unorthodox so there's no direct way to do it without some extra refactoring.